1. Field of the Invention
The present invention relates to a method of growing crystals of a gallium nitride-based compound semiconductor on a substrate consisting of, e.g., sapphire and, more particularly, to a method of growing an epitaxial layer of a gallium nitride-based compound semiconductor with a high crystallinity.
2. Description of the Related Art
Recently, a blue light-emitting device using a gallium nitride-based compound semiconductor, e.g., a compound represented by formula Ga.sub.X Al.sub.1-X N (0.ltoreq..times..ltoreq.1) has attracted attention.
Note that in the following description the compound represented by the above formula is called a gallium nitride-based compound even if X is 0.
As a method of growing crystals of such a gallium nitride-based compound semiconductor, a metalorganic chemical vapor deposition method (to be referred to as an MOCVD method hereinafter) is well known. In this method, an organometallic compound gas is supplied as a reaction gas into a reactor in which a sapphire substrate is placed, and the surface temperature of the substrate is held at a high temperature of 900.degree. C. to 1,100.degree. C., thereby growing an epitaxial layer of compound semiconductor on the substrate. For example, when a GaN epitaxial layer is to be grown, trimethylgallium gas and ammonia gas are used as a Ga-containing gas and an N-containing gas, respectively.
In order to use the grown epitaxial layer of a gallium nitride-based compound semiconductor as a light-emitting device, it is essential to improve the crystallinity of the layer first.
On the surface of the epitaxial layer of GaAlN, formed directly on the sapphire substrate using the MOCVD method, numerous projections and recesses are produced in the form of a hexagonal pyramid-like or hexagonal pillar-like growth pattern. For this reason, crystal surface morphology is very poor in the obtained crystal. Therefore, it is almost impossible to fabricate a blue light-emitting device using a semiconductor crystal layer having numerous projections and recesses on its surface and a very poor surface morphology because the yield is very low.
In order to solve the above problems, each of Appl. Phys. Lett 48, (1986), page 353 and Published Unexamined Japanese Patent Application No. 2-229476, for example, proposes a method of growing an AlN buffer layer on a substrate before growth of an epitaxial layer of a gallium nitride-based compound semiconductor. In this method, an AlN buffer layer with a film thickness of 100 to 500.ANG. is formed on a sapphire substrate at a relatively low growth temperature of 400.degree. C. to 900.degree. C. According to this method, the crystallinity and the surface morphology of a GaAlN epitaxial layer can be improved to some extent by growing the GaAlN epitaxial layer on the AlN layer as a buffer layer.
In this method, however, it is necessary to strictly restrict the growth conditions of the buffer layer. In particular, the film thickness must be strictly set at a very small value of 100 to 500.ANG.. In addition, it is difficult to uniformly form the buffer layer with a predetermined film thickness on the entire surface of a large-area sapphire substrate, e.g., a sapphire substrate about 50 mm in diameter. Therefore, it is difficult to improve the crystallinity and the surface morphology of a GaAlN epitaxial layer formed on the buffer layer with a high yield. Furthermore, the obtained crystallinity is still unsatisfactory to form a practical light-emitting diode or semiconductor laser, i.e., it requires further improvements.
In addition, this method cannot realize a p-n junction sufficient to put a light-emitting diode or the like into practical use. In general, when a light-emitting device is to be fabricated by forming a compound semiconductor layer on a substrate, doping a small amount of an impurity in the compound semiconductor to form an n- or p-type layer and in this manner obtain a p-n junction is known as a very effective means of improving the luminance of the device. However, no blue light-emitting device which realizes a sufficient luminance has been developed yet. The reason for this is that p conductivity type of a semiconductor crystal film cannot be formed.
It is difficult for conventional methods to form a p-type semiconductor crystal film even if p-type impurities such as Zn and Mg may be doped in a GaAlN epitaxial layer, because the crystal film has extremely bad crystallinity. For this reason, conventionally, n-type GaN is grown on a sapphire substrate by, e.g., Halide Vapor Phase Epitaxial (HVPE) crystal growth method, Zn diffusion is performed for the grown layer, and an I layer is formed to fabricate a blue light-emitting device with an MIS structure. However, a light-emitting device fabricated by this method cannot realize a satisfactory luminance efficiency.
In a recently reported technique, in order to form p conductivity type of a semiconductor crystal film, Mg is doped in GaN epitaxial layer, and then an electron beam is radiated on the GaN layer (Oyo Butsuri, 1991, Vol. 60, February, pp. 163 to 166). In this technique, the MOCVD method is used to form an AlN buffer layer with a thickness of 0.02 to 0.05 .mu.m on a sapphire substrate at a low temperature (about 400.degree. C. to 600.degree. C.). Subsequently, the temperature is increased to 1,000.degree. C. An Mg-doped GaN layer is then grown on the AlN buffer layer, and an electron beam is radiated on the surface to form a p-type Mg-doped GaN layer.
This method, however, is still far from a practical level. That is, the characteristics of the disclosed p-type GaN layer are only a maximum free hole concentration (carrier concentration) of 10.sup.17 /cm.sup.3 and a minimum resistivity of 12 .OMEGA..cm.